PD on a ring ordinarily represents palladium. Your ring may be palladium-plated if it's PD prepared above it. HK can be a trademark that means Harriet Kelsall. They style and supply jewellery everywhere in the British isles and to other places also.  Can “JP” also stand for a special region? https://emiliophscl.blogunteer.com/35944855/jp88-no-further-a-mystery